Apparatus for making fluorescent lamps

ABSTRACT

A chamber for processing fluorescent lamp cathodes has a dividing wall with openings receiving the unsealed exhaust tube of the glass stem on which the cathode is supported. Adjacent each opening are conductors connecting the cathode leads to an adjustable source of heating current. While heating the cathodes at scheduled currents, a controlled flow of inert gas, e.g. nitrogen, enters the inlet portion of the chamber on one side of the wall thence through the exhaust tube of each stem past the cathode on the stem in the outlet portion of the chamber on the other side of the wall. The controlled flow of gas allows a carbon dioxide atmosphere at the cathode during reduction of alkaline earth metal carbonate on the cathode to its oxide, while the gas flow provides an otherwise inert environment.

United States Patent Toomey et al. p i a 3,704,927 [451 Dec. 5, 1972 [54] APPARATUS FOR MAKING FLUORESCENT LAMPS [72] Inventors: Charles L. Toomey; Gerald D. Bu-

tler, both of c/o Sylvania Electric Products, Inc., Danvers, Mass. 01923 doned.

521 US. Cl. ..;....1..'..316/30, 316/9, 316/12, 1 316/20 511 Int.Cl ..l-l01j7/20 [58] Field of Search .....3l6/5, 7, 9, l2, 16, 30,32

[56] References Cited UNITED STATES PATENTS 1,670,892 5/1928 Koch ..316/3o 2,020,722 ll/l935 Foulke ..3l6/7X 2,317,754 4/1943 vGorlich.. ..316/5X 3,108,621 10/1963 Harries ..316/30 X Primary Examiner-J. Spencer Overholser Assistant Examiner-R. J. Craig Attorney-Norman J. OMalley and James H. Grover [57] ABSTRACT A chamber for processing fluorescent lamp cathodes has a dividing wall with openings receiving the unsealed exhaust tube of the glass stem on which the cathode is supported. Adjacent each opening are conductors connecting the cathode leads to an adjustable source of heating current. While heating the cathodes at scheduled currents, a controlled flow of inert gas,

e.g. nitrogen, enters the inlet portion of the chamber on one side of the wall thence through the exhaust 1 tube of each stem past the cathode on the stem in the 3 Claims, 4 Drawing Figures PATENTEDBEB 5 m2 INVENTORS A CHARLES TOOMEY BY GERALD D. BUTLER ROBERTS, CUSHMAN 8 GROVER ATTORNEYS APPARATUS FOR MAKING FLUORESCENT LAMPS This is a division of application Ser. No. 562,736,

filed July 5, 1966, now abandoned.

This invention relates to arc discharge-devices, such as fluorescent lamps, and particularly to the manufacture of such devices with improved emissively coated cathodes. f

crease the electron emission of the cathodes they are coated with one or more of. the oxides of the alkaline earth metals barium, strontium and calcium with a small amount, e.g. percent, of zirconium dioxide, appliedforexample as described in United States Pat. No.

In manufacture of the lamp the cathode coating is "applied to a multiply coiled conductor of tungsten, nickel, tantalum or like refractory metal, as a suspension of the alkaline earth carbonates with a binder such as nitrocellulose and a solvent such as amyl acetate. On prior continuous" production lines the cathode,

mounted by lead in wires on a glass.stem,.hasvbeen secured in the lamp envelope by sealing the glass stem to the end of the envelope. After sealing thestem a heating current was passed through the cathode coil causing breakdown of the carbonates to electron emissive oxides: accompanied by formation of carbon monoxide and dioxide. To avoid aresidue of the carbon oxides and gases from theglasssealing flame, the

envelope was flushed'by evacuation. To maintain the rate of the production line this evacuation was necessarily rapidand thorough; Then the inertgas fill was admitted and the envelope finally sealed by fusing exhaust tubes opening through the glass stem. During heating of the cathode coil a further reaction took place between the newly formed alkalineearth oxide and the refractory metal of the cathode conductor with the formation of free, gaseous alkaline earth which was alsopumped out thereby resulting in a reduction of alkaline earth availableas oxideto render the cathode highly electron emissive. In operation of the lamp, subsequent to manufacture, the same reaction occurred with excessive release of free alkaline earth metal in the lamp during the first two to three thousand hours of normal operation. Not only was emissive oxide depleted, shortening the life of the lamp, but also the excessfree alkaline earth deposited on the inner wall of the envelope, discoloring the wall, contaminating the phosphor and reducing light emission.

It is one object of thepresent invention to providea fluorescent lamp,and a way of making such a lamp, in which it is assured that free alkaline earth metals and other contaminants will be substantially absent at the completion of manufacture of the lamp, and that their formation will be minimized duringoperation of the lamp subsequent to manufacture. A further object is to form the cathode oxides at a rate which is independent of the other steps of lamp manufacture such as filling and sealing. t

, According to the invention, apparatus for making an emissive conductor comprises a. chamber having an inert gas inlet and an outlet, means to mount the cathode support in the chambenmeans to adjust the flow of inert gas through the inlet port and chamber, andmeansto connect the cathodes leads to a source of heating current including means to adjust the current, whereby the heated carbonate may be converted to oxide in the presence of carbon oxide untila reaction barrier of a compound of alkalineearth metal and refractory metalis formed between the conductor and coating.

Forthe purpose of illustration typical embodiments of the invention are shown in the accompanying drawing in which: i

.FIG. 1 isan elevation of a fluorescent lamp having a coated cathode; t g V FIG. 2 is an elevation of apparatus, partly broken away, showingapparatus for preparing the cathode coating;

FIG. 3 is anenlarged section on line 3-3- of FIG. 2;

FIG. 4 is a schematic diagram of the electrical circuit ofthe apparatus; and v v t As shown in FIG. 1 a conventional fluorescentlamp comprises an elongate glass envelope 1 with an interior phosphor coating 2. Each end ofthe envelope is closed by glass stem 3 which includes an exhaust tube 4 opening into theinterior of the envelope, and a stem press 5 through which lead wires 6 and 7 enter the envelope. The lead wires are connected to contact pins 8 and 9 insulatively mounted on a base 10 which covers the end of the envelope 1. Inside the envelope the lead wires 6 and 7 support a filamentary electrode 11 electrically connected between the lead wires, and lead wire 6 supports an auxiliary electrode 14. The cathode coil is formed of a refractory metal such as tantalum, nickel or preferably tungsten. r

Prior to sealing the. stem 3 in ,the envelope end, a stem subassembly is prepared with the exhaust tube open at its outer end. An. alkaline earth carbonate cathodecoating is applied over the surface of the coils, and hitherto the stems were sealed in the envelope ends and the envelope was evacuated through an open exhaust tube while heating thecathodes by passing current through the lead wires 6 and 7 and coil 11. The open exhaust tube was sealed by a flame after fillingthe envelope with an inert gas at low pressure.

Shown in FIGS. 2 to 4 is apparatus for breaking down the alkaline earth metal carbonates in a way which prevents substantial loss of alkaline earth metal and results in an improved coated cathode coil. The apparatus comprises a base 21 with a lip .22 provided with a gasket 23. A cover 24 has an edge 26 which rests on the gasket 23 sealing the compartment within the base and cover. The compartment is divided by an insulating wall 27 into an inlet portion or gas manifold 28 connected by an inlet valve 29 to a source of inert gas such as nitrogen, and an outlet portion 29 having an outlet port3l.

Extending through the wall 27 are a plurality of openings 32. As shown in FIG. 3 each opening has a narrow circular bore 33 with a shoulder 34 for receiving and holding the exhaust tube'3. An O-ring 35 seals the gap between the glass exhaust tube and the walls of the openings, and also holds the tube and the stem 3 so that the ends of the leads wires 6 and 7 extending from the stem may be held jammed between the tube 4 and adjacent conductive bus bars 36. s

As shown in FIG. 4, each bus bar 36 is fixed on the insulating wall on either side of an opening 33. The lead wires 6 and 7 are jammed in electrical contact with respective bars by inserting the tube 4 in a wall opening 33 with the wires loosed in the space between bars, and then rotating the stem and tube until the wires individually jam between the tube and a bar. The bus bars are arranged in three series connected by wires 37, 38, 39 and 40 to an autotransformer T having alternating 1 current line input terminals C..The autotransformer T has an adjustable tap t for varying the current supplied to the three series of bus bars. In each series is an ammeter A measuring the current through each series. When each opening adjacent the bus bars of a series holds a stem with the lead wires jammed against opposed bus bars, the electrodes on the stems are connected in the series.

With stems mounted in all the wall openings and the cover 24 sealed on the base 21, the cathodes on the stems are processed as follows. Nitrogen or other inert gas is admitted through the valve 29 to the inlet, manifold chamber portion 28. Thence the gas flows through each exhaust tube 4 past the cathode on each steminto the outlet chamber, whence the gas is exhausted through the cover opening 31. After flushing atmosphere from the chamber portions and during continued flow of the inert gas, heating current is supplied through the cathodes, the amount of current and the rate of flow of gas being adjusted to maintain gaseous carbon oxides present adjacent the cathodes in quantities dependent on the amount of refractory metal used in the cathode.

As one example of a cathode for use in a 48 inch VHO fluorescent lamp, a triple coiled filament is formed as follows. A fine, 1.5 mg. tungsten wire is wound at 338 t.p.i. on a145.8 mg. tungsten mandrel in parallel with a 7 mil. molybdenum mandrel (later removed by acid). The resulting primary coil is wound at 50 t.p.i. on a 14.25 molybdenum mandrel (also later removed). About 4.5 turns of the resulting secondary coil are wound at 17.5 t.p.i. on a 50 mil. steel mandrel, which is then removed. The total, clean coil weight of the triple coil and 45.8 mg. tungsten-mandrel is 48 to 56 mg. of tungsten. The triple coil is then coated with 23 mg. of a mixture of the combination of alkaline steps while nitrogen is flowingcontinuously at 1 cubic 1 foot per hour.

Step 1 Step 2 Step 3 Step 4 Current 0.5 1.0 1.5 2.0

Time 5 During processing the following primary reactions take place when barium carbonate is the principal cathode coating ingredient. First, the barium carbonate I is broken down, by heating current through the cathode, into barium oxide and carbon dioxide. The barium carbonate reacts with the refractory metal of v the cathode, preferably tungsten, to yield barium tung-- state and carbon monoxide. And, most importantly, in the presence of carbon dioxide maintained by low rate of nitrogen flow the reduced barium oxide reacts with tungsten to produce barium tungstate, and incidentally carbon monoxide. As compared with prior processes, this latter reaction is continued until it produces at the interface between the tungsten cathode and barium carbonatecoating a layer of barium tungstate which forms a barrier to further reaction between the reduced bariumoxide coating and the tungsten cathode conductor. Further, the the presence of carbon dioxide substantially inhibits a reaction present in prior process, namely the reaction between barium oxide and tungsten releasing free barium which is removed by flushing or evacuation and thereby removed from the emissive cathode coating.

In contrast with prior processes in which the barium tungstate was produced in incidental amounts and with barium loss, the present process produces a complete reaction barrier between coating and cathode. The processed cathode may be stored in a normal atmospheric environment, and later sealed within a lamp envelope by conventional exhausting and sealing procedures. Thereafter the interface barrier prevents the presence-and further production of free barium in the carbon-dioxide-free fill of the finished lamp, whereas, in the operation of prior lamps, continued reaction between barium oxide in the coating and the tungsten cathode released barium to reduce the available emissive oxide and discolor and contaminate the phosphor coating of the lamp envelope.

In a lamp so processed the barium is conserved for its principal function of providing electron emissive oxide, increasing the life of the lamp and decreasing its discoloration and loss of phosphor emission. In life tests, very high output (VHO) lamps have been increased 40 per cent in life, and their discoloration has been decreased by one-third.

Although barium oxide and tungsten reactions have been described as examples, other alkaline earth metal oxides and refractory metals will produce lamps with longer lives in comparison with prior lamps whose cathodes are processed at the same time as final filling, stem sealing and evacuation under conditions con trolled by requirements of finishing rather than cathode processing. Although control by batch processing of cathode mounts has been described, continuous cathode processing on the lamp production line can be carried out under control conditions separated from the final sealing.

Thus, while one desirable embodiment of the invention has herein been disclosed by way of example, it is to be understood that the invention is broadly inclusive of any and all modifications falling within the terms of the appended claims.

We claim:

1. Apparatus for processing arefractory metal cathode conductor mounted on a support with lead wires connected thereto and said conductor having a coating of alkaline earth metal carbonate, which comprises,

a chamber having an inert gas inlet and an outlet, .means for mounting the cathode support in the chamber including means to direct the gas past the cathode, means for connecting the cathode leads to asource of heating current, I whereby the heated carbonate may be converted to oxide in the presence of carbon oxide until a reaction barrier of a compound of alkaline earth metal oxide and refractory metal is formed between the conductor and coating, without production of free alkaline earth metal, and wherein said mounting means comprises an insulating wall dividing said chamber into an inlet portion and an outlet portion, said wall having openings to receive and hold said cathode conductor support, and said connecting means includes conductor means adjacent said openings and extending inlet and outlet chamber portions substantial 3. Apparatus according to claim '1 wherein said cathode support comprises a hollow open ended tube leading toward said cathode, and said mounting means comprises a wall dividing said chamber into an inlet and an outlet portion, said wall having an opening receiving said tube with its ends in the inlet and outlet chambers respectively, whereby the insert gas is directed past the cathode adjacent the end of the tube in the outlet chamber. 

1. Apparatus for processing a refractory metal cathode conductor mounted on a support with lead wires connected thereto and said conductor having a coating of alkaline earth metal carbonate, which comprises, a chamber having an inert gas inlet and an outlet, means for mounting the cathode support in the chamber including means to direct the gas past the cathode, means for connecting the cathode leads to a source of heating current, whereby the heated carbonate may be converted to oxide in the presence of carbon oxide until a reaction barrier of a compound of alkaline earth metal oxide and refractory metal is formed between the conductor and coating, without substantial production of free alkaline earth metal, and wherein said mounting means comprises an insulating wall dividing said chamber into an inlet portion and an outlet portion, said wall having openings to receive and hold said cathode conductor support, and said connecting means includes conductor means adjacent said openings and extending between a plurality of said openings for contacting said lead wires thereby to connect a plurality of said cathodes to said source.
 2. Apparatus according to claim 1 wherein said cathode conductor support comprises a hollow tube and each said aperture receives said tube snugly to hold said tube in said wall in gas communication with said inlet and outlet chamber portions
 3. Apparatus according to claim 1 wherein said cathode support comprises a hollow open ended tube leading towarD said cathode, and said mounting means comprises a wall dividing said chamber into an inlet and an outlet portion, said wall having an opening receiving said tube with its ends in the inlet and outlet chambers respectively, whereby the insert gas is directed past the cathode adjacent the end of the tube in the outlet chamber. 